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Have you tried Mr Google?
In a nutshell, (I'm assuming a Windows solution):
Your gateway will have an address like 192.168.1.1 with netmask 255.255.255.0.
If the gateway routes to the Internet, install Comodo firewall on it.
Disable Windows "firewall" on all LAN PCs.
Ban all users from installing apps from unscanned sources or install antivirus solution on every PC.
Assign IPs from the same subnet to the LAN PCs, e.g. 192.168.1.2 etc. or set up the gateway to do DHCP.
All your game patches, driver updates, utils etc. can be on a shared directory on the gateway and mounted on network drives on the LAN PCs.